1. Field of the Invention
The present invention relates to a spatial light modulator and a spatial light modulating apparatus which can execute real time light modulation and real time image storage.
2. Description of the Related Art
A spatial light modulator (hereinafter referred to as SLM) functions to write a two dimensional pattern (e.g., an image) therein with write light and read the pattern with another light (read light). This can ensure processes such as amplification of image light, a process involving a threshold value, inversion, incoherent-coherent conversion between read light and write light and wavelength conversion.
A conventional SLM is constituted using a nematic liquid crystal 101 (see Optical Engineering, Vol. 17, p. 371, 1978). The SLM 100 employs a photodiode structure of CdS/CdTe as a photosensitive layer 102, and the nematic liquid crystal 101 is filled neighboring on a dielectric mirror 103. For that portion irradiated with write light 104, due to its diode characteristic, a voltage applied to the liquid crystal 101 increases over a threshold value of a liquid crystal. As a result, the liquid crystal axis having been aligned along the face of glass substrate 105 is aligned in the voltage-applied direction. Accordingly, the plane of polarization does not rotate by linearly polarized read light 106. For that part not irradiated with the write light 104, as the liquid crystal axis remains aligned along the face of the glass substrate 105, the plane of polarization of the read light 106 rotates. It is therefore possible to read out a pattern corresponding to a written pattern through an analyzer (not shown).
Another conventional SLM is known which uses a ferroelectric liquid crystal (hereinafter referred to as FLC) having a higher response speed, as shown in FIGS. 2 and 3 (refer to SPIE, Vol. 684, p. 60, 1986 and Mat. Res. Soc. Symp. Proc., Vol. 118, p. 405, 1988). According to a conventional SLM 110 shown in FIG. 2, a BSO (bismuth silicon oxide) is used for a photosensitive layer 111, and the thin film is adhered on one glass substrate 112, with a FLC 113 being filled between two glass substrates. According to a SLM 120 shown in FIG. 3, an a-Si (amorphous silicon) film having a p-i-n diode structure is used as a photosensitive layer 121, and a FLC 123 is filled between glass substrates 124 neighboring on a reflector 122.
The above-described conventional SLMs have the following problems.
(1) According to the structure of the SLM 100 shown in FIG. 1, the response speed of the nematic liquid crystal 101 is slow, so that the SLM 100 operates at a speed of as high as several tens of milliseconds. Further, without a memory capability, the SLM 100 should execute pattern writing and reading simultaneously.
(2) According to the structure of the SLM 110 shown in FIG. 2, the response speed of the BSO of the photosensitive layer 111 is slow, so that the SLM 110 likewise operates at a speed of as high as several tens of milliseconds. As the BSO is insensitive to red light, it is necessary to use a specific type of light, e.g., argon laser light, as write light. Further, since a thin film of BSO should be adhered on the top of the glass substrate 112 to provide a SLM, the fabrication of the SLM is not easy and it is difficult to control the uniform thickness of the SLM. Because of no memory capability, the SLM 110 should always be applied with a pulse voltage, does not operate on a single non-repetitive pulse, and provides a low contrast due to attenuation.
(3) According to the structure of the SLM 120 shown in FIG. 3 in which an a-Si film having a p-i-n diode structure is used as the photosensitive layer 121, because of the diode characteristic, a pattern can be written only when a negative voltage is applied and write light 125 needs a high intensity of 50 mW/cm.sup.2, thus impairing the sensitivity. Likewise, due to no memory capability, the SLM 120 should always be applied with a pulse voltage, does not operate on a single non-repetitive pulse, and provides a low contrast due to attenuation.
Another prior art is known which is an optical recording device having a combination of a photo-conductive layer and a BLC and a recording method utilizing the same as disclosed in Published Unexamined Japanese Patent Application No. 59-216126. FIG. 4 illustrates the structure of the prior art; referring to this diagram, 401 and 401' are transparent substrates, 402 and 402' are transparent conductive layers, 403 is a photoconductive layer, 404 is a light shielding layer, 405 is a dielectric mirror, 406 and 406' are thin insulating layers, 407 is a FLC layer, 408 and 409 are DC voltage sources, 410 is a switch, 411 is write light, 412 is a polarization plate, and 413 is read light. The DC voltage sources 408 and 409 are selectively used by the switch 410 so that a bi-stable switching is effected by application of a positive voltage or a negative voltage. With this structure, since no alignment film exists in the device, the initial aligning direction cannot be stably defined for a long period of time, the contrast will be deteriorated. According to the recording method that keeps applying a DC voltage at the time of image erasing and writing the life of the liquid crystal is shortened because of different voltages applied to the FLC and different time intervals of voltage application at the time of erasing and writing.